Reading from ISO9660 in U-Boot

[Heinrich Schuchardt]

On 9/23/23 00:13, Simon Glass wrote:
> Hi Heinrick & Bin,
>
> I'd like to be able to figure out in U-Boot what OS is on a USB stick.
> For example, with the Ubuntu installer, I can boot it (through grub),
> but I cannot see how to read anything useful from the USB stick that
> would indicate that it is Ubuntu, what version it is, etc.
>
> Does U-Boot need an ISO9660-filesystem driver for that? Is there any other way?
>
> With Debian I can see the actual files (linux and initrd) , so it is a
> bit easier.
>
> Regards,
> SImon

The format of the Ubuntu images depends on the architecture. Let's
assume that you relate to amd64
(https://cdimage.ubuntu.com/daily-live/current/mantic-desktop-amd64.iso).

This is a hybrid image, which can be either read as ISO 9660 or as a GPT
partitioned image. This is the gdisk output:

Number  Start (sector)    End (sector)  Size       Code  Name
    1              64        10049451   4.8 GiB     0700  ISO9660
    2        10049452        10059487   4.9 MiB     EF00  Appended2
    3        10059488        10060087   300.0 KiB   0700  Gap1

You can mount partition 2 as FAT file system in Linux:

$ sudo kpartx mantic-desktop-amd64.iso -a -v
[sudo] password for zfsdt:
add map loop1p1 (252:4): 0 10049388 linear 7:1 64
add map loop1p2 (252:5): 0 10036 linear 7:1 10049452
add map loop1p3 (252:6): 0 600 linear 7:1 10059488

$ sudo mount /dev/mapper/loop1p2 /mnt

$ mount
/dev/mapper/loop1p2 on /mnt type vfat
(rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)

The problem is upstream U-Boot is that the partition drivers are scanned
alphabetically: a_efi, dos, iso

If we change this to a_iso, b_efi, dos, we can read the ISO image:

=> host bind 0 /tmp/mantic-desktop-amd64.iso
=> part list host 0

Partition Map for HOST device 0  --   Partition Type: ISO

Part   Start     Sect x Size Type
   1     3684        4    512 U-Boot
   2 10049452    10036    512 U-Boot

=> ls host 0:2
             EFI/

=> load host 0:2 $kernel_addr_r EFI/boot/bootx64.efi
960472 bytes read in 0 ms

Unfortunately the sandbox crashes when executing the EFI binary.

So let't try qemu-x86_64_defconfig:

qemu-system-x86_64 -m 3G -nographic -bios u-boot.rom \
-drive if=none,file=/tmp/mantic-desktop-amd64.iso,format=raw,id=VIRTIO1 \
-device virtio-blk,drive=VIRTIO1

And voilá your are in GRUB.

As it does not find the /boot directory let's go to the GRUB console:

grub> root=(cd0,gpt1)
grub> linux /casper/vmlinuz root=/dev/vda1 efi=debug console=/dev/ttyS0
grub> initrd /casper/initrd
grub> boot
EFI stub: Loaded initrd from LINUX_EFI_INITRD_MEDIA_GUID device path

I have no clue why it stops here.

Best regards

Heinrich